Ahodges/doc/cohorts

@explorerleslie , @nasthagiri , @andy-armstrong , @gwprice , @jimabramson , @dan-f , @cahrens, @srpearce  , @WatsonEmily , @mhoeber 
The cohorts/cohort_config and cohorts/cohorts_overview files and images are pretty solid. Questions occasionally in comments.
I'll work on the section describing moderating discussions in a cohorted universe next.
@cahrens , see note above to @explorerleslie . We use this term throughout the Building & Running guide to indicate a course that you want to view in the LMS. I don't know why, but we do not use "LMS" anywhere in the documentation, we use this terminology instead.